Small internal combustion engines provide convenience and power to those who use hand-held or portable power equipment, particularly lawn and garden equipment, such as chain saws, lawn mowers, flexible line trimmers, leaf blowers and vacuums, and lawn edgers. Most portable equipment is powered by two-stroke internal combustion engines that are normally aspirated, crankcase scavenged, and spark-ignited. Indeed, this type of two-stroke engine is very well suited to power forestry, lawn and garden equipment. It delivers more power for its weight than a four-stroke engine. Thus, the portable equipment can be made smaller and lighter. It is also a very simple design and, thus, is less expensive to manufacture and maintain and more reliable.
On the other hand, two-stroke engines generally burn fuel less efficiently and emit more pollutants than four-stroke engines. This is in part due to the nature of two-stroke engines, in which fresh fuel and air charge is taken into a cylinder on the same stroke as exhaust is discharged. Scavenging is the process of eliminating, after combustion, gases and filling a cylinder with a fresh charge during the same stroke, usually by displacing the gases with a flow of fresh charge. Scavenging is less than perfect in practice. Fresh charge often mixes with the burned gas, resulting in a mixture of fresh and burned gases that is combustible. Combustion of the mixture is less efficient, reducing power output and creating more pollutant gases. Some of the fresh charge is not trapped in the cylinder and exits with gases to an exhaust system. The lost charge, referred to as scavenge loss, reduces fuel efficiency and increases hydrocarbon emissions.
Two-stroke engines that power portable tools have not been subject to emission control standards applicable to motor vehicles. However, the State of California has passed legislation that will impose rather stringent restrictions on pollutant emission from internal combustion engines in all classes of application, including portable equipment. The United States Environmental Protection Agency has proposed exhaust emission regulations that would apply to portable equipment.
Various methods have been proposed to help to improve the performance of two-stroke engines and to reduce pollutant emissions in two-stroke engines. Many of these methods are explained by Franz J. Laimbock in "The Potential of Small Loop-Scavenged Spark-Ignition Single Cylinder Two-Stroke Engines." These improvement efforts focus on scavenging and trapping efficiency, fuel injection, "lean" air/fuel mixtures, treatment of exhaust with catalysts, and intake and exhaust systems. However, it is not clear which methods will result in sufficient reduction in pollutant emissions to meet legal limits and prove feasible for portable power tools.
Several methods have been proposed to improve scavenging and trapping efficiency. In "loop" scavenging, a scavenging stream of air and fuel enters the cylinder in a direction opposite the exhaust window in the cylinder and flows in a loop. The loop tends to displace exhaust gas in a systematic manner, improving scavenging. The longer the distance travelled by the loop the more effective the trapping. In a "cross-scavenging" design, the scavenge and exhaust ports are on opposite sides of the cylinder. The piston head includes a ramp that deflects the fresh charge upward and away from the exhaust port.
In a "uniflow" engine, scavenging gases flow only in one direction in the cylinder. The scavenge port and the exhaust port are placed opposite each other, generally in the direction of the cylinder axis. Scavenging losses are reduced by the long distance between the scavenge port and the exhaust port. Uniflow engines are understandably well suited to long-stroke engines, such as large-capacity, supercharged, marine diesel engines. In these engines, scavenge losses are air only, as fuel is injected after the exhaust port is closed. In these long-stroke engines, the exhaust port is usually located at the end of the cylinder opposite the scavenge port and controlled with a cam-operated popper valve.
Alternatively, a uniflow engine, may be constructed by bending the cylinder into a "U" shape, in effect creating two cylinders connected by a common combustion chamber. On one cylinder is a scavenge port, controlled by the timing edge of a piston; on the other cylinder is an exhaust port controlled by the timing edge of a second piston. The scavenge piston and the exhaust piston are connected by a common combustion chamber. Each piston moves in opposition with respect to the other.
However, uniflow engines have the disadvantage of being more complicated than conventionally scavenged or loop scavenged engines used in portable power equipment, and thus they are less desirable. A remotely located exhaust valve increases the complexity, size and cost of the engine and raises a number of technical problems such as design of a mechanism to operate the exhaust valve and lubrication of that mechanism, especially when the position of the engine frequently changes during use and storage. Similarly, multiple pistons also tend to increase the size and complexity of the engine.
An alternate approach to improve exhaust emissions of small-displacement internal combustion engines that power portable equipment is to dispense with the two-stroke engine and go with a four stroke engine. A four-stroke engine has significantly less power than a comparably sized two-stroke engine, is more complicated mechanically due the necessity of valves, and is much more difficult to lubricate, especially when it is being held in different positions during use and storage. Nevertheless, as stated, recently issued U.S. Pat. No. 5,213,074, issued to Juragawa et at. and assigned to Ryobi Limited, a four-stroke engine is superior to a two-stroke in that it has relatively clean exhaust, is less noisy and has better fuel economy. Juragawa thus proposes a four-stroke engine with an improved lubrication system to power a portable tool.